Hi cornelistromp,
Yeah its the infamous "macejowski’s glaive" , personally to me it looks like some sort of a glaive-polearm (hence the name i guess) that was hack off at the shaft. There were heated debates about the origin of the weapon on several other "arms n armour" oriented forums , but to no avail since no such "living" speciment was unearthed. Who knows maybye it was just the authors imagination
(note also the man-at-arms that is hacked in half with his guts out , seems like an exeggaration to make the painting/battle more impresive)